The invention relates to a video signal processing system which converts video signals from a format of sequential scanning to that of interlaced scanning.
In a computer system such as a personal computer or an engineering workstation, the video signal has a format of sequential scanning and more horizontal scanning lines than the television broadcasting system such as NTSC, PAL, or SECAM which has a format of interlaced scanning. Accordingly, the image which is formed according to the video signal outputted from the computer system and shown on the standard television monitor includes flickers generally.
The principle of generation of flickering will be described below with reference to FIGS. 4A, 4B and 4C. FIG. 4A shows typical video signals in the format of sequential scanning. As shown in the figure, the horizontal scanning lines are scanned in series in order of 1, 2, 3, 4 . . . from top to bottom. The video signals of one frame are stored in a frame memory, for example. After that, the odd horizontal scanning lines are read out from the memory in order of 1, 3, 5 . . . and they constitute the first field as shown in FIG. 4B. Similarly, the second field comprises the even lines of 2, 4, 6 . . . as shown in FIG. 4C. By this reading of the odd and even lines, the video signals of one frame comprising two fields in the format of interlaced scanning are obtained.
However, where there exists an isolated luminance signal which is shown typically as a shaded area in FIG. 4A only in one specific horizontal scanning line (in the 3rd line in the embodiment), the luminance signal is present in the first field as shown in FIG. 4B; but not in the second field as shown in FIG. 4C. Therefore, since in NTSC system the frame time period is one-thirtieth seconds, in other words the field time period is one-sixtieth seconds, the luminance signal is present every one-thirtieth seconds and is observed as flickers by the viewers. Similarly in the PAL or SECAM system it is present every one-fiftieth seconds.